There are two known types of color proofing methods: the overlay type and the surprint type. One example of the overlay type method is disclosed in U.S. Pat. No. 3,136,637. In this overlay type proofing system, colored sheets exposed through respective proper color separation negatives and developed are superposed upon each other to produce a color proofing sheet.
This overlay type method has the advantages that it is quite simple and that it can serve as a progressive proof by combining two or three colors in registration. However, it has the disadvantage that the superposed synthetic resin sheets tend to darken the color test sheet and incident light is reflected by several sheets to provide luster, and, as a result, the impression of the color test sheet thus prepared becomes different from copies actually obtained by a conventional printing press.
In the known surprint type of color proofing method, a color proofing sheet is prepared by successively superposing differently colored layers upon each other on a single sheet. In this method, various toners are applied to a single, common, opaque base, or light-sensitive layers of corresponding colors are provided successively on an opaque base.
This type of method is described in, for example, U.S. Pat. Nos. 3,671,236 and 3,136,637.
This surprint type method has the advantage that color density is not influenced by the synthetic resin base. This method more closely resembles the actual printing process and eliminates the color distortion caused upon superposing the sheets. However, it has the defect that it produces weak images.
Various processes for producing copies of an image embodying photopolymerization and thermal transfer techniques are also known. Examples thereof are described in U.S. Pat. Nos. 3,060,023, 3,060,024, 3,060,025, 3,481,736 and 3,607,264. In these processes, a photopolymerizable layer coated on a suitable support is imagewise exposed through a color separation negative.
The surface of the exposed layer is then pressed into contact with the image-receiving layer of a separate element, and at least one of the elements is heated to a temperature above the transfer temperature of the unexposed portions of the layer. The two elements are then separated, whereupon the thermally transferrable, unexposed image areas of the composite transfer to the image-receiving element by the heat of the composite. If the element is not precolored, the tacky unexposed image may now be selectively colored with a suitable toner which preferentially adheres to the unpolymerized material. The lamination, exposure, and development steps are carried out for the respective colors in sequence, thus making the process time-consuming. In addition, the above process also produces only a weak image, and, hence, it is necessary, for practical use, to provide a protective layer after image formation.
U.S. Pat. No. 3,721,557 describes a method of transferring colored images, which involves a stripping layer between a light-sensitive layer and a carrier. When the light-sensitive layer is exposed to actinic light and developed, more soluble portions thereof are selectively removed to produce a visible image. The image-carrying carrier is pressed against a suitable image-receiving member and, subsequently, the carrier is stripped to accomplish image transfer.
A fresh layer of adhesive is applied to the image-receiving member for each subsequent transfer. The extra two steps (1) of coating a stripping layer between the light-sensitive layer and the carrier and (2) application of an adhesive after each image transfer are time consuming and costly. In addition, the image buildup in a four-color proof is so thick that it produces moire patterns and the image is so distorted that the proof cannot by any cleaner or truer than an overlay proof.
Most of the above described processes are positive working systems. Some require expensive and sophisticated transfer equipment and depend on sequential lamination (pressure and heat), exposure, and transfer (or delamination of a polymer) processes for each color used. This makes the total process very time consuming.
Further, U.S. Pat. No. 4,304,836 discloses a color proofing system applicable to both the overlay type method and the surprint type method.
In this color proofing system,
(1) various color separation images are formed on transparent carrier sheets, PA0 (2) an adhesive layer is provided on an intermediate carrier, PA0 (3) a paper and the intermediate carrier are superposed upon each other to transfer the adhesive layer to the paper, PA0 (4) one image-carrying sheet is pressed into contact with the image-receiving sheet covered with an adhesive layer to transfer the image to the adhesive layer in the state of being embedded in the adhesive layer, and PA0 (5) each additional image is registered on the preceding image and press-transferred in the same manner.
This method is quite advantageous in that it enables one to conduct both the simple overlay type method and the surprint type method providing copies more resembling actual print copies. However, it has the defect that the finally obtained image has weak scratch resistance and some tackiness due to the softness and tackiness of the adhesive layer.
In order to raise scratch resistance and reduce tackiness, the adhesive layer must be made harder and less tacky. This, however, requires extremely high temperature or pressure image embedding, resulting in a deterioration of the transfer fidelity of the image. A problem always encountered with the above described surprint type method is also weakness of image strength. Halftone dot images tend to drop unless processing is carefully conducted or the images produced are handled with care.
An object of the present invention is to overcome the problems with the above conventional methods and to provide a color proofing sheet which can be used in a simple process for both the overlay type method and the surprint type method which yields final images having high strength.